Basic Propeller Parts

The first step to understanding propellers and how they work is familiarizing yourself with the basic parts of a propeller. (Figure 2-1)

A. Blade Tip


The maximum reach of the blade from the center of the propeller hub. It separates the leading edge from the trailing edge.

B. Leading Edge


That part of the blade nearest the boat, which first cuts through the water. It extends from the hub to the tip.

C. Trailing Edge


That part of the blade farthest from the boat. The edge from which the water leaves the blade. It extends from the tip to the hub (near the diffuser ring on through-hub exhaust propellers).

D. Cup


A small curve or lip on the trailing edge of the blade, permitting the propeller to hold water better and normally adding about 1/2" (12.7 mm) to 1" (25.4 mm) of pitch.

E. Blade Face


That side of the blade facing away from the boat, known as the positive pressure side of the blade.

F. Blade Back


The side of the blade facing the boat, known as the negative pressure (or suction) side of the blade.

G. Blade Root


The point at which the blade attaches to the hub.

H. Inner Hub


This contains the Flo-Torq rubber hub or Flo-Torq II Delrin® Hub System (Figures 2-2 and 2-3). The forward end of the inner hub is the metal surface which generally transmits the propeller thrust through the forward thrust hub to the propeller shaft and in turn, eventually to the boat.

I. Outer Hub


For through-hub exhaust propellers. The exterior surface is in direct contact with the water. The blades are attached to the exterior surface. Its inner surface is in contact with the exhaust passage and with the ribs which attach the outer hub to the inner hub.

J. Ribs


For through-hub exhaust propellers. The connections between the inner and outer hub. There are usually three ribs, occasionally two, four, or five. The ribs are usually either parallel to the propeller shaft ("straight"), or parallel to the blades ("helical").

K. Flo-Torq™ Shock-Absorbing Rubber Hub


Rubber molded to an inner splined hub to protect the propeller drive system from impact damage and to flex when shifting the engine, to relieve the normal shift shock that occurs between the gear and clutch mechanism - generally used with low horsepower applications.

Flo-Torq II Shock-Absorbing Delrin® Hub
Patented hub system designed to resist slippage, yet flex during engine shifting and cushions the drivetrain upon underwater impact. The Flo-Torq II system makes Mercury Propellers compatible with almost all marine engines.

L. Diffuser Ring


Aids in reducing exhaust back pressure and in preventing exhaust gas from feeding back into propeller blades

M. Exhaust Passage


For through-hub exhaust propellers. The hollow area between the inner hub and the outer hub through which engine exhaust gases are discharged into the water. In some stern drive installations using a through-transom exhaust system, this passage carries air.

N. Performance Vent System (PVS)


PVS, a patented Mercury ventilation system, allows the boater to custom tune the venting of the propeller blades for maximum planing performance. On acceleration, exhaust is drawn out of the vent hole located behind each blade. When the next propeller blade strikes this aerated water, less force is required to push through this water allowing the engine RPM to rise more rapidly. Water flows over the vent holes once the boat is on plane sending exhaust through the exhaust passage. Varying the size of the exhaust holes engine RPM can be controlled, outboards perform better with venting and stern drives typically require less venting if any at all.